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<a name="Qualifiers"></a>
<h3 class="section">4.10 Qualifiers</h3>

<ul>
<li> <cite>What constitutes an access to an object that has volatile-qualified
type (C90 6.5.3, C99 and C11 6.7.3).</cite>

<p>Such an object is normally accessed by pointers and used for accessing
hardware.  In most expressions, it is intuitively obvious what is a read
and what is a write.  For example
</p>
<div class="smallexample">
<pre class="smallexample">volatile int *dst = <var>somevalue</var>;
volatile int *src = <var>someothervalue</var>;
*dst = *src;
</pre></div>

<p>will cause a read of the volatile object pointed to by <var>src</var> and store the
value into the volatile object pointed to by <var>dst</var>.  There is no
guarantee that these reads and writes are atomic, especially for objects
larger than <code>int</code>.
</p>
<p>However, if the volatile storage is not being modified, and the value of
the volatile storage is not used, then the situation is less obvious.
For example
</p>
<div class="smallexample">
<pre class="smallexample">volatile int *src = <var>somevalue</var>;
*src;
</pre></div>

<p>According to the C standard, such an expression is an rvalue whose type
is the unqualified version of its original type, i.e. <code>int</code>.  Whether
GCC interprets this as a read of the volatile object being pointed to or
only as a request to evaluate the expression for its side effects depends
on this type.
</p>
<p>If it is a scalar type, or on most targets an aggregate type whose only
member object is of a scalar type, or a union type whose member objects
are of scalar types, the expression is interpreted by GCC as a read of
the volatile object; in the other cases, the expression is only evaluated
for its side effects.
</p>
<p>When an object of an aggregate type, with the same size and alignment as a
scalar type <code>S</code>, is the subject of a volatile access by an assignment
expression or an atomic function, the access to it is performed as if the
object&rsquo;s declared type were <code>volatile S</code>.
</p>
</li></ul>

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